Engine exhaust catalysts containing palladium-gold

ABSTRACT

An emission control catalyst that exhibits improved CO and HC reduction performance includes a supported platinum-based catalyst, and a supported palladium-gold catalyst. The two catalysts are coated onto different layers, zones, or monoliths of the substrate for the emission control catalyst such that the platinum-based catalyst encounters the exhaust stream before the palladium-gold catalyst. Zeolite may be added to the emission control catalyst as a hydrocarbon absorbing component to boost the oxidation activity of the palladium-gold catalyst.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/867,335, filed Nov. 27, 2006, which is hereinincorporated by reference.

This application claims priority to the same provisional application asU.S. patent application Ser. No. 11/624,116, filed Jan. 17, 2007, U.S.patent application Ser. No. 11/624,128, filed Jan. 17, 2007, and U.S.patent application Ser. No. 11/942,712, filed Nov. 20, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to supportedcatalysts containing precious group metals and, and more particularly,to engine exhaust catalysts containing palladium and gold, and methodsof production thereof.

2. Description of the Related Art

Many industrial products such as fuels, lubricants, polymers, fibers,drugs, and other chemicals would not be manufacturable without the useof catalysts. Catalysts are also essential for the reduction ofpollutants, particularly air pollutants created during the production ofenergy and by automobiles. Many industrial catalysts are composed of ahigh surface area support material upon which chemically active metalnanoparticles (i.e., nanometer sized metal particles) are dispersed. Thesupport materials are generally inert, ceramic type materials havingsurface areas on the order of hundreds of square meters/gram. This highspecific surface area usually requires a complex internal pore system.The metal nanoparticles are deposited on the support and dispersedthroughout this internal pore system, and are generally between 1 and100 nanometers in size.

Processes for making supported catalysts go back many years. One suchprocess for making platinum catalysts, for example, involves thecontacting of a support material such as alumina with a metal saltsolution such as hexachloroplatinic acid in water. The metal saltsolution “impregnates” or fills the pores of the support during thisprocess. Following the impregnation, the support containing the metalsalt solution would be dried, causing the metal salt to precipitatewithin the pores. The support containing the precipitated metal saltwould then be calcined (typically in air) and, if necessary, exposed toa reducing gas environment (e.g., hydrogen or carbon monoxide) forfurther reduction to form metal particles. Another process for makingsupported catalysts involves the steps of contacting a support materialwith a metal salt solution and reducing the metal ions to metalparticles in situ using suitable reducing agents.

Supported catalysts are quite useful in removing pollutants from vehicleexhausts. Vehicle exhausts contain harmful pollutants, such as carbonmonoxide (CO), unburned hydrocarbons (HC), and nitrogen oxides (NOx),that contribute to the “smog-effect” that have plagued majormetropolitan areas across the globe. Catalytic converters containingsupported catalysts and particulate filters have been used to removesuch harmful pollutants from the vehicle exhaust. While pollution fromvehicle exhaust has decreased over the years from the use of catalyticconverters and particulate filters, research into improved supportedcatalysts has been continuing as requirements for vehicle emissioncontrol have become more stringent and as vehicle manufacturers seek touse less amounts of precious metal in the supported catalysts to reducethe total cost of emission control.

The prior art teaches the use of supported catalysts containingpalladium and gold as good partial oxidation catalysts. As such, theyhave been used extensively in the production of vinyl acetate in thevapor phase by reaction of ethylene, acetic acid and oxygen. See, e.g.,U.S. Pat. No. 6,022,823. As for vehicle emission control applications,U.S. Pat. No. 6,763,309 speculates that palladium-gold might be a goodbimetallic candidate for increasing the rate of NO decomposition. Thedisclosure, however, is based on a mathematical model and is notsupported by experimental data. There is also no teaching in this patentthat a palladium-gold system will be effective in treating vehicleemissions that include CO and HC.

SUMMARY OF THE INVENTION

The present invention provides emission control catalysts for treatingemissions that include CO and HC, and methods for producing the same.The engine may be a vehicle engine, an industrial engine, or generally,any type of engine that burns hydrocarbons.

An emission control catalyst according to embodiments of the presentinvention includes a supported platinum-based catalyst and a supportedpalladium-gold catalyst. The two catalysts are coated onto differentlayers, zones, or monoliths of the substrate for the emission controlcatalyst such that the platinum-based catalyst encounters the exhauststream before the palladium-gold catalyst. Zeolite may be added to theemission control catalyst as a hydrocarbon absorbing component to boostthe oxidation activity of the palladium-gold catalyst.

The inventors have enabled the use of supported catalysts comprisingpalladium and gold species as emission control catalysts by overcomingthe problem which they have discovered through tests that HC speciespresent in the exhaust inhibit the oxidation activity of such catalysts.With the present invention, such HC inhibition effects are reducedsufficiently by exposing the exhaust to the platinum-based catalystbefore the palladium-gold catalyst and/or by adding a hydrocarbonabsorbing material, so that the oxidation activity of the palladium-goldcatalyst can be improved and the overall catalytic activity of theemission control catalyst can be boosted to effective levels. Theinventors have confirmed through vehicle performance tests that theemission control catalysts according to embodiments of the presentinvention perform as well as platinum-palladium catalysts in reducing COand HC emissions from a vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIGS. 1A-1D are schematic representations of different engine exhaustsystems in which embodiments of the present invention may be used.

FIG. 2 is an illustration of a catalytic converter with a cut-awaysection that shows a substrate onto which emission control catalystsaccording to embodiments of the present invention are coated.

FIGS. 3A-3D illustrate different configurations of a substrate for anemission control catalyst.

FIG. 4 is a flow diagram illustrating the steps for preparing anemission control catalyst according to an embodiment of the presentinvention.

FIG. 5 is a flow diagram illustrating the steps for preparing anemission control catalyst according to another embodiment of the presentinvention.

DETAILED DESCRIPTION

In the following, reference is made to embodiments of the invention.However, it should be understood that the invention is not limited tospecific described embodiments. Instead, any combination of thefollowing features and elements, whether related to differentembodiments or not, is contemplated to implement and practice theinvention. Furthermore, in various embodiments the invention providesnumerous advantages over the prior art. However, although embodiments ofthe invention may achieve advantages over other possible solutionsand/or over the prior art, whether or not a particular advantage isachieved by a given embodiment is not limiting of the invention. Thus,the following aspects, features, embodiments and advantages are merelyillustrative and are not considered elements or limitations of theappended claims except where explicitly recited in the claims. Likewise,reference to “the invention” shall not be construed as a generalizationof any inventive subject matter disclosed herein and shall not beconsidered to be an element or limitation of the appended claims exceptwhere explicitly recited in the claims.

FIGS. 1A-1D are schematic representations of different engine exhaustsystems in which embodiments of the present invention may be used. Thecombustion process that occurs in an engine 102 produces harmfulpollutants, such as CO, various hydrocarbons, particulate matter, andnitrogen oxides (NOx), in an exhaust stream that is discharged through atail pipe 108 of the exhaust system.

In the exhaust system of FIG. 1A, the exhaust stream from an engine 102passes through a catalytic converter 104, before it is discharged intothe atmosphere (environment) through a tail pipe 108. The catalyticconverter 104 contains supported catalysts coated on a monolithicsubstrate that treat the exhaust stream from the engine 102. The exhauststream is treated by way of various catalytic reactions that occurwithin the catalytic converter 104. These reactions include theoxidation of CO to form CO₂, burning of hydrocarbons, and the conversionof NO to NO₂.

In the exhaust system of FIG. 1B, the exhaust stream from the engine 102passes through a catalytic converter 104 and a particulate filter 106,before it is discharged into the atmosphere through a tail pipe 108. Thecatalytic converter 104 operates in the same manner as in the exhaustsystem of FIG. 1A. The particulate filter 106 traps particulate matterthat is in the exhaust stream, e.g., soot, liquid hydrocarbons,generally particulates in liquid form. In an optional configuration, theparticulate filter 106 includes a supported catalyst coated thereon forthe oxidation of NO and/or to aid in combustion of particulate matter.

In the exhaust system of FIG. 1C, the exhaust stream from the engine 102passes through a catalytic converter 104, a pre-filter catalyst 105 anda particulate filter 106, before it is discharged into the atmospherethrough a tail pipe 108. The catalytic converter 104 operates in thesame manner as in the exhaust system of FIG. 1A. The pre-filter catalyst105 includes a monolithic substrate and supported catalysts coated onthe monolithic substrate for the oxidation of NO. The particulate filter106 traps particulate matter that is in the exhaust stream, e.g., soot,liquid hydrocarbons, generally particulates in liquid form.

In the exhaust system of FIG. 1D, the exhaust stream passes from theengine 102 through a catalytic converter 104, a particulate filter 106,a selective catalytic reduction (SCR) unit 107 and an ammonia slipcatalyst 110, before it is discharged into the atmosphere through a tailpipe 108. The catalytic converter 104 operates in the same manner as inthe exhaust system of FIG. 1A. The particulate filter 106 trapsparticulate matter that is in the exhaust stream, e.g., soot, liquidhydrocarbons, generally particulates in liquid form. In an optionalconfiguration, the particulate filter 106 includes a supported catalystcoated thereon for the oxidation of NO and/or to aid in combustion ofparticulate matter. The SCR unit 107 is provided to reduce the NO_(x)species to N₂. The SCR unit 107 may be ammonia/urea based or hydrocarbonbased. The ammonia slip catalyst 110 is provided to reduce the amount ofammonia emissions through the tail pipe 108. An alternativeconfiguration places the SCR unit 107 in front of the particulate filter106.

Alternative configurations of the exhaust system includes the provisionof SCR unit 107 and the ammonia slip catalyst 110 in the exhaust systemof FIG. 1A or 1C, and the provision of just the SCR unit 107, withoutthe ammonia slip catalyst 110, in the exhaust system of FIGS. 1A, 1B or1C.

As particulates get trapped in the particulate filter within the exhaustsystem of FIGS. 1B, 1C or 1D, it becomes less effective and regenerationof the particulate filter becomes necessary. The regeneration of theparticulate filter can be either passive or active. Passive regenerationoccurs automatically in the presence of NO₂. Thus, as the exhaust streamcontaining NO₂ passes through the particulate filter, passiveregeneration occurs. During regeneration, the particulates get oxidizedand NO₂ gets converted back to NO. In general, higher amounts of NO₂improve the regeneration performance, and thus this process is commonlyreferred to as NO₂ assisted oxidation. However, too much NO₂ is notdesirable because excess NO₂ is released into the atmosphere and NO₂ isconsidered to be a more harmful pollutant than NO. The NO₂ used forregeneration can be formed in the engine during combustion, from NOoxidation in the catalytic converter 104, from NO oxidation in thepre-filter catalyst 105, and/or from NO oxidation in a catalyzed versionof the particulate filter 106.

Active regeneration is carried out by heating up the particulate filter106 and oxidizing the particulates. At higher temperatures, NO₂assistance of the particulate oxidation becomes less important. Theheating of the particulate filter 106 may be carried out in various waysknown in the art. One way is to employ a fuel burner which heats theparticulate filter 106 to particulate combustion temperatures. Anotherway is to increase the temperature of the exhaust stream by modifyingthe engine output when the particulate filter load reaches apre-determined level.

The present invention provides catalysts that are to be used in thecatalytic converter 104 shown in FIGS. 1A-1D, or generally as catalystsin any vehicle emission control system, including as a diesel oxidationcatalyst, a diesel filter catalyst, an ammonia-slip catalyst, an SCRcatalyst, or as a component of a three-way catalyst. The presentinvention further provides a vehicle emission control system, such asthe ones shown in FIGS. 1A-1D, comprising an emission control catalystcomprising a monolith and a supported catalyst coated on the monolith.

FIG. 2 is an illustration of a catalytic converter with a cut-awaysection that shows a substrate 210 onto which supported metal catalystsare coated. The exploded view of the substrate 210 shows that thesubstrate 210 has a honeycomb structure comprising a plurality ofchannels into which washcoats containing supported metal catalysts areflowed in slurry form so as to form coating 220 on the substrate 210.

FIGS. 3A-3D illustrate different embodiments of the present invention.In the embodiment of FIG. 3A, coating 220 comprises two washcoat layers221, 223 on top of substrate 210. Washcoat layer 221 is the bottom layerthat is disposed directly on top of the substrate 210 and contains metalparticles having palladium and gold in close contact (also referred toas “palladium-gold metal particles”). Washcoat layer 223 is the toplayer that is in direct contact with the exhaust stream and containsmetal particles having platinum alone or in close contact with anothermetal species such as palladium (also referred to as“platinum-containing metal particles”). Based on their positionsrelative to the exhaust stream, washcoat layer 223 encounters theexhaust stream before washcoat layer 221.

In the embodiment of FIG. 3B, coating 220 comprises three washcoatlayers 221, 222, 223 on top of substrate 210. Washcoat layer 221 is thebottom layer that is disposed directly on top of the substrate 210 andincludes palladium-gold metal particles. Washcoat layer 223 is the toplayer that is in direct contact with the exhaust stream and includesplatinum-containing metal particles. Washcoat layer 222 is the middlelayer that is disposed in between washcoat layers 221, 223. The middlelayer is provided to minimize the interaction between the Pt and Pd—Aucomponents. The middle layer may be a blank support or may containzeolites, rare earth oxides, or inorganic oxides. Based on theirpositions relative to the exhaust stream, washcoat layer 223 encountersthe exhaust stream before washcoat layers 221, 222, and washcoat layer222 encounters the exhaust stream before washcoat layer 221.

In the embodiment of FIG. 3C, the substrate 210 is a single monoliththat has two coating zones 210A, 210B. A washcoat includingplatinum-containing metal particles is coated onto a first zone 210A anda washcoat including palladium-gold metal particles is coated onto asecond zone 210B.

In the embodiment of FIG. 3D, the substrate 210 includes first andsecond monoliths 231, 232, which are physically separate monoliths. Awashcoat including platinum-containing metal particles is coated ontothe first monolith 231 and a washcoat including palladium-gold metalparticles is coated onto the second monolith 232.

All of the embodiments described above include a palladium-gold catalystin combination with a platinum-based catalyst. The weight ratio ofpalladium to gold in the palladium-gold catalyst is about 0.05:1 to20:1, preferably from about 0.5:1 to about 2:1. The palladium-goldcatalyst may be promoted with bismuth or other known promoters. Theplatinum-based catalyst may be a platinum catalyst, a platinum-palladiumcatalyst, a platinum catalyst promoted with bismuth or other nowpromoters, or other platinum-based catalysts (e.g., Pt—Rh, Pt—Ir, Pt—Ru,Pt—Au, Pt—Ag, Pt—Rh—Ir, Pt—Ir—Au, etc.). The preferred embodimentsemploy a platinum-palladium catalyst as the platinum-based catalyst. Theweight ratio of platinum to palladium in this catalyst is about 0.05:1to 20:1, preferably from about 2:1 to about 4:1.

In addition, the platinum-based catalyst is situated so that itencounters the exhaust stream prior to the palladium-gold catalyst. Bypositioning the platinum-based catalyst relative to the palladium-goldcatalyst in this manner, the inventors have discovered that HCinhibition effects on the oxidation activity of the palladium-goldcatalyst are reduced to sufficient levels so that the overall catalyticperformance is improved. In the embodiments of FIGS. 3A and 3B, theplatinum-based catalyst is included in the top layer 223 and thepalladium-gold catalyst is included in the bottom layer 221. In theembodiment of FIG. 3C, the platinum-based catalyst is included in thefirst zone 210A and the palladium-gold catalyst is included in thesecond zone 210B. In the embodiment of FIG. 3D, the platinum-basedcatalyst is included in the first monolith 231 and the palladium-goldcatalyst is included in the second monolith 232.

In additional embodiments of the present invention, a hydrocarbonabsorbing material is added to the emission control catalyst.Preferably, the hydrocarbon absorbing material is added to the emissioncontrol catalyst so that it encounters exhaust stream prior to thepalladium-gold catalyst. By positioning the hydrocarbon absorbingmaterial relative to the palladium-gold catalyst in this manner, theinventors have discovered that HC inhibition effects on the oxidationactivity of the palladium-gold catalyst are reduced to sufficient levelsso that the overall catalytic performance is improved. In theconfiguration shown in FIG. 3A, the hydrocarbon absorbing material maybe included in the top layer 223. In the configuration shown in FIG. 3B,the hydrocarbon absorbing material may be included in the middle layer222 or the top layer 223. In the configuration shown in FIG. 3C, thehydrocarbon absorbing material may be included in the first zone 210A.In the configuration shown in FIG. 3D, the hydrocarbon absorbingmaterial may be included in the front monolith 231. In the examplesprovided below, a hydrocarbon absorbing material is zeolite. Zeolite maybe a beta zeolite, ZSM-5 zeolite, and mixtures of the two, with orwithout other types of zeolites, in any weight ratio.

In other embodiments of the present invention, any of the washcoatlayers or zones, or monoliths may include rare-earth oxides, such ascerium(IV) oxide (CeO₂) and ceria-zirconia (CeO₂—ZrO₂).

FIG. 4 is a flow diagram that illustrates the steps for preparing anemission control catalyst according to an embodiment of the presentinvention using the substrate 210. In step 410, a first supportedcatalyst, e.g., supported palladium-gold catalyst, is prepared isaccordance with known methods or with the methods described in theexamples provided below. In step 412, a second supported catalyst, e.g.,supported platinum-based catalyst, is prepared in accordance with knownmethods or with the methods described in the examples provided below. Amonolithic substrate, such as substrate 210 shown in FIG. 2 (ormonolithic substrates 231, 232 shown in FIG. 3D) is provided in step414. Exemplary monolithic substrates include those that are ceramic(e.g., cordierite), metallic, or silicon carbide based. In step 416, thefirst supported catalyst in powder form are mixed in a solvent to form awashcoat slurry, and the washcoat slurry is coated as the bottom layerof the substrate 210 or onto a rear zone or rear monolith of thesubstrate 210. In step 418, the second supported catalyst in powder formare mixed in a solvent to form a washcoat slurry, and the washcoatslurry is coated as the top layer of the substrate 210 or onto a frontzone or front monolith of the substrate 210. Optionally, zeolite orzeolite mixture including one or more of beta zeolite, ZSM-5 zeolite,and other types of zeolites is added to the washcoat slurry before thewashcoat slurry is coated in step 418.

FIG. 5 is a flow diagram that illustrates the steps for preparing anemission control catalyst according to another embodiment of the presentinvention using the substrate 210. In step 510, a first supportedcatalyst, e.g., supported palladium-gold catalyst, is prepared isaccordance with known methods or with the methods described in theexamples provided below. In step 512, a second supported catalyst, e.g.,supported platinum-based catalyst, is prepared in accordance with knownmethods or with the methods described in the examples provided below. Amonolithic substrate, such as substrate 210 shown in FIG. 2, is providedin step 514. Exemplary monolithic substrates include those that areceramic (e.g., cordierite), metallic, or silicon carbide based. In step516, the first supported catalyst in powder form are mixed in a solventto form a washcoat slurry, and the washcoat slurry is coated as thebottom layer of the substrate 210. In step 517, zeolite or zeolitemixture is added to a solvent to form a washcoat slurry and thiswashcoat slurry is coated as the middle layer of the substrate 210. Instep 518, the second supported catalyst in powder form are mixed in asolvent to form a washcoat slurry, and the washcoat slurry is coated asthe top layer of the substrate 210.

The data representing vehicle performance of the various embodiments ofthe present invention are provided in Tables 1 and 2.

TABLE 1 CO HC emissions emissions Example Bottom Layer Middle Layer TopLayer (g/km) (g/km) 1 PtPd (2.8%:1.4% Beta zeolite at PtPd (2.8%:1.4%0.366 0.079 by weight) at 57.5 g/ft³ 0.5 g/in³ by weight) at 57.5 g/ft³2 PtPd (2.8%:1.4% Beta zeolite and PtPd (2.8%:1.4% 0.332 0.066 byweight) at 57.5 g/ft³ ZSM-5 zeolite by weight) at 57.5 g/ft³ (1:1 byweight) 3 PdAu (1.7%:2.0% Beta zeolite and PtPd (3.0%:0.75% 0.296 0.049Test A by weight) at 65 g/ft³ ZSM-5 zeolite by weight) at 65.0 g/ft³(1:1 by weight) 3 PdAu (1.7%:2.0% Beta zeolite and PtPd (3.0%:0.75%0.296 0.057 Test B by weight) at 65 g/ft³ ZSM-5 zeolite by weight) at65.0 g/ft³ (1:1 by weight)

TABLE 2 CO emissions HC emissions Example Front Brick Rear Brick (g/km)(g/km) 4 PtPd (2.0%:1.0% by PtPd (2.0%:1.0% by 0.143 0.0539 weight) at120 g/ft³ weight) at 120 g/ft³ 5 PtPd (2.0%:1.0% by PdAu (1.7%:2.0% by0.146 0.0474 weight) at 120 g/ft³ weight) at 175 g/ft³ 6 PtPd(3.0%:0.75% by PdAu (1.7%:2.0% by 0.121 0.0505 weight) at 130 g/ft³weight) at 130 g/ft³ 7 PtPd (2.0%:1.0% by PdAu (1.7%:2.0% by 0.1230.0385 weight) at 150 g/ft³ weight) at 130 g/ft³

The data presented in Tables 1 and 2 above reflect the vehicle testperformance for seven catalysts with equal precious group metal cost(assuming cost basis of Pt:Pd:Au of 4:1:2) that have been engine agedfor 20 hours (with a two-mode cycle using fuel injection to get catalystbed temperatures of about 650° C.). The CO and HC emissions weremeasured from the tail pipe of a light-duty diesel vehicle (model year2005) using bag data from the standard European MVEG test. Examples ofTable 1 were tested under low engine out temperatures (around 150° C. to300° C.) and Examples of Table 2 were tested under high engine outtemperatures (around 200° C. to 350° C.). Also, in Examples 1-3,catalysts were coated on a cordierite substrate with a diameter of 5.66inches and length of 2.5 inches. In Examples 4-7, catalysts were coatedon a pair of cordierite substrates, each with a diameter of 5.66 inchesand length of 1.25 inches.

Table 1 presents data for emission control catalysts having a tri-layerconfiguration (see FIG. 3B). Example 1 represents a benchmark emissioncontrol catalyst and includes metal particles having platinum andpalladium in close contact (also referred to as “platinum-palladiummetal particles”) having a weight ratio of 2.8%:1.4% in the bottom layerand the top layer. The middle layer comprises beta zeolite. Example 2also represents a benchmark emission control catalyst and has the samecomposition as Example 1 except the middle layer comprises a zeolitemixture of beta zeolite and ZSM-5 zeolite at a weight ratio of 1:1.Example 3 represents an emission control catalyst according to anembodiment of the present invention and includes palladium-gold metalparticles having a weight ratio of 1.7%:2.0% in the bottom layer andplatinum-palladium metal particles having a weight ratio of 3.0%:0.75%in the top layer. The middle layer comprises a zeolite mixture of betazeolite and ZSM-5 zeolite at a weight ratio of 1:1. Relative to thebenchmark emission control catalysts of Examples 1 and 2, a reduction inboth HC and CO emissions has been observed with the emission controlcatalyst of Example 3.

Table 2 presents data for emission control catalysts having a dual-brickconfiguration (see FIG. 3D). Example 4 represents a benchmark emissioncontrol catalyst and includes platinum-palladium metal particles havinga weight ratio of 2.0%:1.0% in the front brick and the rear brick.Examples 5, 6 and 7 represent emission control catalysts according toembodiments of the present invention, each of which includespalladium-gold metal particles. Example 5 includes platinum-palladiummetal particles having a weight ratio of 2.0%:1.0% in the front brickand palladium-gold metal particles having weight ratio of 1.7%:2.0% inthe rear brick. Example 6 includes platinum-palladium metal particleshaving a weight ratio of 4.0%:1.0% in the front brick and palladium-goldmetal particles having weight ratio of 1.7%:2.0% in the rear brick.Example 7 includes platinum-palladium metal particles having a weightratio of 2.0%:1.0% in the front brick and palladium-gold metal particleshaving weight ratio of 1.7%:2.0% in the rear brick. Both bricks inExample 7 used a washcoat slurry with approximately 28% ceria-zirconiaadded in (the rest was the precious group metal and alumina powder).Relative to the benchmark emission control catalyst of Example 4, areduction in HC emissions and similar or better CO oxidation performancehave been observed with the emission control catalysts of Examples 5, 6and 7.

The preparation methods for Examples 1-7 were as follows:

Preparation of a 1.7% Pd, 2.0% Au supported PdAu Catalyst.

Lanthanum-stabilized alumina (578 g, having a surface area of ˜200m²g⁻¹) and 2940 mL of de-ionized water (>18MΩ) were added to a 5 Lplastic beaker and magnetically stirred at about 500 rpm. The pHmeasured was 8.5 and the temperature measured was 25° C. After 20minutes, Pd(NO₃)₂ (67.8 g of 14.8% aqueous solution) was gradually addedover a period of 10 min. The pH measured was 4.3. After stirring for 20minutes, a second metal, HAuCl₄ (24 g dissolved in 50 mL of de-ionizedwater), was added over a period of 5 min. The pH was 4.0 and thetemperature of the metal-support slurry was 25° C. The metal-supportslurry was stirred for an additional 30 min. In a second vessel, NaBH₄(29.4 g) and NaOH (31.1 g) were added to N₂H₄ (142 mL of 35% aqueoussolution) and stirred until the mixture became clear. This mixtureconstituted the reducing agent mixture. The metal-support slurry andreducing agent mixture were combined continuously using two peristalticpumps. The two streams were combined using a Y joint connected to aVigreux column to cause turbulent mixing. The reaction product leavingthe mixing chamber, i.e., the Vigreux column, was pumped into anintermediate vessel of smaller volume and continuously stirred. Theproduct in the intermediate vessel was continuously pumped into a largervessel, i.e., 5 L beaker, for residence and with continued stirring. Theentire addition/mixing process lasted about 30 min. The resultingproduct slurry was stirred in the larger vessel for an additional periodof 1 h. The final pH was 11.0 and the temperature was 25° C. The productslurry was then filtered using vacuum techniques via Buchner funnelsprovided with a double layer of filter paper having 3 μm porosity. Thefilter cake was then washed with about 20 L of de-ionized water inseveral approximately equal portions. Thereafter, the washed cake wasdried at 110° C., ground to a fine powder using a mortar and pestle, andsubsequently calcined at 500° C. for 2 h, with a heating rate of 8° C.min⁻¹. This supported PdAu catalyst powder (1.7% Pd, 2.0% Au) was usedin preparing Examples 3, 5, 6 and 7.

Preparation of a 2.8% Pt, 1.4% Pd Supported Catalyst.

To 10 L of de-ionized H₂O was added 1940 g of La-stabilized alumina(having a BET surface area of ˜200 m² g⁻¹) followed by stirring for 30minutes at room temperature. To this slurry was added 490.6 g ofPt(NO₃)₂ solution (12.23% Pt(NO₃)₂ by weight), followed by stirring atroom temperature for 60 minutes. Acrylic acid (750 mL, 99% purity) wasthen added into the system over 12 minutes and the resulting mixture wasallowed to continue stirring at room temperature for 2 hours. The solidLa-doped alumina supported Pt catalyst was separated from the liquid viafiltration, dried at 120° C. for 2 hours, ground into a fine powder, andcalcined in air for 2 hours at a temperature of 500° C. (heated at 8° C.min⁻¹) to give a 3% Pt material.

To 9.25 L of de-ionized H₂O was added 1822 g of the above 3% Pt materialfollowed by stirring for 20 minutes at room temperature. To this slurrywas added 194.4 g of Pd(NO₃)₂ solution (14.28% Pd(NO₃)₂ by weight),followed by stirring at room temperature for 60 minutes. An aqueousascorbic acid solution (930 g in 4.5 L of de-ionized H₂O) was then addedover 25 minutes followed by stirring for 60 minutes. The solid La-dopedalumina supported PtPd catalyst was separated from the liquid viafiltration, dried at 120° C. for 2 hours, ground into a fine powder, andcalcined in air for 2 hours at a temperature of 500° C. (heated at 8° C.min⁻¹) to give a 3% Pt, 1.5% Pd material. This material was diluted to2.8% Pt, 1.4% Pd via addition of blank La-doped alumina support and thediluted mixture was used in preparing Examples 1 and 2.

Preparation of a 2.0% Pt, 1.0% Pd Supported Catalyst.

To 10 L of de-ionized H₂O was added 2000 g of La-stabilized alumina(having a BET surface area of ˜200 m² g⁻¹) followed by stirring for 30minutes at room temperature. To this slurry was added 327.1 g ofPt(NO₃)₂ solution (12.23% Pt(NO₃)₂ by weight), followed by stirring atroom temperature for 60 minutes. Acrylic acid (500 mL, 99% purity) wasthen added into the system over 12 minutes and the resulting mixture wasallowed to continue stirring at room temperature for 2 hours. The solidLa-doped alumina supported Pt catalyst was separated from the liquid viafiltration, dried at 120° C. for 2 hours, ground into a fine powder, andcalcined in air for 2 hours at a temperature of 500° C. (heated at 8° C.min⁻¹) to give a 2% Pt material.

To 9.5 L of de-ionized H₂O was added 1900 g of the above 2% Pt materialfollowed by stirring for 20 minutes at room temperature. To this slurrywas added 135.3 g of Pd(NO₃)₂ solution (14.28% Pd(NO₃)₂ by weight),followed by stirring at room temperature for 60 minutes. An aqueousascorbic acid solution (647.2 g in 3.5 L of de-ionized H₂O) was thenadded over 25 minutes followed by stirring for 60 minutes. The solidLa-doped alumina supported PtPd catalyst was separated from the liquidvia filtration, dried at 120° C. for 2 hours, ground into a fine powder,and calcined in air for 2 hours at a temperature of 500° C. (heated at8° C. min⁻¹) to give a 2% Pt, 1% Pd material. This material was used inpreparing Examples 4, 5 and 7.

Preparation of a 3.0% Pt, 0.75% Pd Supported Catalyst

To 10 L of de-ionized H₂O was added 2000 g of La-stabilized alumina(having a BET surface area of ˜200 m² g⁻¹) followed by stirring for 30minutes at room temperature. To this slurry was added 654.2 g ofPt(NO₃)₂ solution (12.23% Pt(NO₃)₂ by weight), followed by stirring atroom temperature for 60 minutes. Acrylic acid (500 mL, 99% purity) wasthen added into the system over 12 minutes and the resulting mixture wasallowed to continue stirring at room temperature for 2 hours. The solidLa-doped alumina supported Pt catalyst was separated from the liquid viafiltration, dried at 120° C. for 2 hours, ground into a fine powder, andcalcined in air for 2 hours at a temperature of 500° C. (heated at 8° C.min⁻¹) to give a 4% Pt material.

To 9.5 L of de-ionized H₂O was added 3800 g of the above 4% Pt materialfollowed by stirring for 20 minutes at room temperature. To this slurrywas added 135.3 g of Pd(NO₃)₂ solution (14.28% Pd(NO₃)₂ by weight),followed by stirring at room temperature for 60 minutes. An aqueousascorbic acid solution (647.2 g in 3.5 L of de-ionized H₂O) was thenadded over 25 minutes followed by stirring for 60 minutes. The solidLa-doped alumina supported PtPd catalyst was separated from the liquidvia filtration, dried at 120° C. for 2 hours, ground into a fine powder,and calcined in air for 2 hours at a temperature of 500° C. (heated at8° C. min⁻¹) to give a 4% Pt, 1% Pd material. This material was thendiluted to 3.0% Pt, 0.75% Pd via addition of blank La-doped aluminasupport and the diluted mixture was used in preparing Examples 3 and 6.

EXAMPLE 1 Tri-layer: PtPd (at 57.5 g/ft³) 1st Layer, Beta Zeolite 2ndLayer, PtPd (at 57.5 g/ft³) 3rd Layer

The supported PtPd catalyst powder (2.8% Pt, 1.4% Pd) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d₅₀ range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto a round cordierite monolith(Corning, 400 cpsi, 5.66 inches×2.5 inches), dried at 120° C. andcalcined at 500° C. to give the first layer of the multi-layer coatedmonolith, such that the PtPd loading was ˜57.5 g/ft³.

Then, beta zeolite was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d₅₀ range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the zeolite washcoat slurry was coated onto the cordierite monolith(with the first layer of PtPd), dried at 120° C. and calcined at 500° C.to give the second layer of the multi-layer coated monolith. The zeolitemixture comprises about 20% of the total washcoat loading.

Then, the supported PtPd catalyst powder (2.8% Pt, 1.4% Pd) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d₅₀ range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto the cordierite monolith (withthe first layer of PtPd and the second layer of zeolite), dried at 120°C. and calcined at 500° C. to give the third layer of the multi-layercoated monolith, such that the PtPd loading was ˜57.5 g/ft³.

The multi-layer coated monolith was canned according to methods known inthe art and tested using a certified testing facility on a light-dutydiesel vehicle, as described above.

EXAMPLE 2 Tri-layer: PtPd (at 57.5 g/ft³) 1st Layer, Zeolite Mixture 2ndLayer, PtPd (at 57.5 g/ft³) 3rd Layer

The supported PtPd catalyst powder (2.8% Pt, 1.4% Pd) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d₅₀ range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto a round cordierite monolith(Corning, 400 cpsi, 5.66 inches×2.5 inches), dried at 120° C. andcalcined at 500° C. to give the first layer of the multi-layer coatedmonolith, such that the PtPd loading was ˜57.5 g/ft³.

Then, equal weight amounts of a beta zeolite and a ZSM-5 zeolite werecombined and made into a washcoat slurry via addition to de-ionizedwater, milling to an appropriate particle size (typically with a d₅₀range from 3 to 7 μm), and pH adjustment to give an appropriateviscosity for washcoating. According to methods known in the art, thezeolite washcoat slurry was coated onto the cordierite monolith (withthe first layer of PtPd), dried at 120° C. and calcined at 500° C. togive the second layer of the multi-layer coated monolith. The zeolitemixture comprises about 20% of the total washcoat loading.

Then, the supported PtPd catalyst powder (2.8% Pt, 1.4% Pd) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d₅₀ range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto the cordierite monolith (withthe first layer of PtPd and the second layer of zeolite), dried at 120°C. and calcined at 500° C. to give the third layer of the multi-layercoated monolith, such that the PtPd loading was ˜57.5 g/ft³.

The multi-layer coated monolith was canned according to methods known inthe art and tested using a certified testing facility on a light-dutydiesel vehicle, as described above.

EXAMPLE 3 PdAu (at 65 g/ft³) 1st Layer, Zeolite Mixture 2nd Layer, PtPd(at 65 g/ft³) 3rd Layer

The supported PdAu catalyst powder (1.7% Pd, 2.0% Au) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d₅₀ range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto a round cordierite monolith(Corning, 400 cpsi, 5.66 inches×2.5 inches), dried at 120° C. andcalcined at 500° C. to give the first layer of the multi-layer coatedmonolith, such that the PdAu loading was ˜65 g/ft³.

Then, equal weight amounts of a beta zeolite and a ZSM-5 zeolite werecombined and made into a washcoat slurry via addition to de-ionizedwater, milling to an appropriate particle size (typically with a d₅₀range from 3 to 7 μm), and pH adjustment to give an appropriateviscosity for washcoating. According to methods known in the art, thezeolite washcoat slurry was coated onto the cordierite monolith (withthe first layer of PtPd), dried at 120° C. and calcined at 500° C. togive the second layer of the multi-layer coated monolith. The zeolitemixture comprises about 20% of the total washcoat loading.

Then, the supported PtPd catalyst powder (3.0% Pt, 0.75% Pd) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d₅₀ range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto the cordierite monolith (withthe first layer of PdAu and the second layer of zeolite), dried at 120°C. and calcined at 500° C. to give the third layer of the multi-layercoated monolith, such that the PtPd loading was ˜65 g/ft³.

The multi-layer coated monolith was canned according to methods known inthe art and tested using a certified testing facility on a light-dutydiesel vehicle, as described above.

EXAMPLE 4 Multi-brick: Pt/Pd at 120 g/ft³

The supported PtPd catalyst powder (2.0% Pt, 1.0% Pd) prepared above wasmade into a washcoat slurry via addition to de-ionized water, milling toan appropriate particle size (typically with a d₅₀ range from 3 to 7μm), and pH adjustment to give an appropriate viscosity for washcoating.According to methods known in the art, the washcoat slurry was coatedonto both the front brick and the rear brick of a round cordieritemonolith (each brick: Corning, 400 cpsi, 5.66 inches×1.25 inches), driedat 120° C. and calcined at 500° C. to give the final coated monolithwith a precious metal (Pt+Pd) loading of 120 g/ft³. The coated monolithwas canned according to methods known in the art and tested using acertified testing facility on a light-duty diesel vehicle, as describedabove.

EXAMPLE 5 Multi-brick: Pt/Pd (at 120 g/ft³) Front and PdAu (at 175g/ft³) Rear

The supported PtPd catalyst powder (2.0% Pt, 1.0% Pd) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d₅₀ range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto a round cordierite monolith(Corning, 400 cpsi, 5.66 inches×1.25 inches), dried at 120° C. andcalcined at 500° C. to give the final coated monolith with a preciousmetal loading of 120 g/ft³ PtPd. This represented the front brick of atwo brick system.

In addition, the supported PdAu catalyst powder (1.7% Pd, 2.0% Au)prepared as described above was made into a washcoat slurry via additionto de-ionized water, milling to an appropriate particle size (typicallywith a d₅₀ range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto a round cordierite monolith(Corning, 400 cpsi, 5.66 inches×1.25 inches), dried at 120° C. andcalcined at 500° C. to give the final coated monolith with a preciousmetal loading of 175 g/ft³ PdAu. This represented the outlet brick of atwo brick system.

The coated PtPd monolith (front brick) and the coated PdAu monolith(rear brick) were then canned according to methods known in the art suchthat the front brick was closest to the engine (and hence would beexposed to the exhaust first), and tested using a certified testingfacility on a light-duty diesel vehicle, as described above.

EXAMPLE 6 Multi-brick: Pt/Pd (at 130 g/ft³) Front and PdAu (at 130g/ft³) Rear

The supported PtPd catalyst powder (3.0% Pt, 0.75% Pd) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d₅₀ range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto a round cordierite monolith(Corning, 400 cpsi, 5.66 inches×1.25 inches), dried at 120° C. andcalcined at 500° C. to give the final coated monolith with a preciousmetal loading of 130 g/ft³ PtPd. This represented the front brick of atwo brick system.

In addition, the supported PdAu catalyst powder (1.7% Pd, 2.0% Au)prepared as described above was made into a washcoat slurry via additionto de-ionized water, milling to an appropriate particle size (typicallywith a d₅₀ range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto a round cordierite monolith(Corning, 400 cpsi, 5.66 inches×1.25 inches), dried at 120° C. andcalcined at 500° C. to give the final coated monolith with a preciousmetal loading of 130 g/ft³ PdAu. This represented the outlet brick of atwo brick system.

The coated PtPd monolith (front brick) and the coated PdAu monolith(rear brick) were then canned according to methods known in the art suchthat the front brick was closest to the engine (and hence would beexposed to the exhaust first), and tested using a certified testingfacility on a light-duty diesel vehicle, as described above.

EXAMPLE 7 Multi-brick: Pt/Pd (at 150 g/ft³) Front and PdAu (at 130g/ft³) Rear

The supported PtPd catalyst powder (2.0% Pt, 1.0% Pd) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d₅₀ range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. Ceria-zirconia was added to thiswashcoat slurry so that ceria-zirconia represented about 28% by weight.According to methods known in the art, the washcoat slurry was coatedonto a round cordierite monolith (Corning, 400 cpsi, 5.66 inches×1.25inches), dried at 120° C. and calcined at 500° C. to give the finalcoated monolith with a precious metal loading of 150 g/ft³ PtPd. Thisrepresented the front brick of a two brick system.

In addition, the supported PdAu catalyst powder (1.7% Pd, 2.0% Au)prepared as described above was made into a washcoat slurry via additionto de-ionized water, milling to an appropriate particle size (typicallywith a d₅₀ range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. Ceria-zirconia was added to thiswashcoat slurry so that ceria-zirconia represented about 28% by weight.According to methods known in the art, the washcoat slurry was coatedonto a round cordierite monolith (Corning, 400 cpsi, 5.66 inches×1.25inches), dried at 120° C. and calcined at 500° C. to give the finalcoated monolith with a precious metal loading of 130 g/ft³ PdAu. Thisrepresented the outlet brick of a two brick system.

The coated PtPd monolith (front brick) and the coated PdAu monolith(rear brick) were then canned according to methods known in the art suchthat the front brick was closest to the engine (and hence would beexposed to the exhaust first), and tested using a certified testingfacility on a light-duty diesel vehicle, as described above.

While particular embodiments according to the invention have beenillustrated and described above, those skilled in the art understandthat the invention can take a variety of forms and embodiments withinthe scope of the appended claims.

1. An emission control catalyst for treating an engine exhaustcomprising a first catalytically active zone and a second catalyticallyactive zone, wherein the first catalytically active zone is positionedto encounter the engine exhaust before the second catalytically activezone, and wherein the first catalytically active zone includes a firstsupported catalyst comprising platinum-containing metal particlessupported on an oxide carrier and the second catalytically active zoneincludes a second supported catalyst consisting essentially ofpalladium-gold metal particles supported on an oxide carrier, whereinthe second supported catalyst has palladium to gold weight ratio rangeof about 0.5:1 to about 1:0.5.
 2. The emission control catalystaccording to claim 1, further comprising a substrate having a honeycombstructure with gas flow channels, wherein the first supported catalystand the second supported catalyst are coated on the walls of the gasflow channels.
 3. The emission control catalyst according to claim 2,wherein multiple layers are coated on the walls of the gas flowchannels, and a bottom layer that is directly above the walls of the gasflow channels includes the second supported catalyst and a top layerthat is positioned to be in direct contact with the engine exhaustincludes the first supported catalyst.
 4. The emission control catalystaccording to claim 3, wherein the first supported catalyst comprisesplatinum-palladium metal particles.
 5. The emission control catalystaccording to claim 3, wherein the top layer further includes zeolite. 6.The emission control catalyst according to claim 3, wherein the multiplelayers further include a middle layer between the bottom layer and thetop layer.
 7. The emission control catalyst according to claim 6,wherein the middle layer comprises zeolite.
 8. The emission controlcatalyst according to claim 1, further comprising a substrate having anupstream zone and a downstream zone, the upstream zone being positionedto encounter the engine exhaust before the downstream zone, wherein thefirst supported catalyst is coated on the upstream zone of the substrateand the second supported catalyst is coated on the downstream zone ofthe substrate.
 9. The emission control catalyst according to claim 1,further comprising an upstream monolith and a downstream monolith, theupstream monolith being positioned to encounter the engine exhaustbefore the downstream monolith, wherein the first supported catalyst iscoated on the upstream monolith and the second supported catalyst iscoated on the downstream monolith.
 10. An emission control catalyst fortreating an engine exhaust comprising a substrate having a firstwashcoat metal particles comprising platinum and a second washcoatcontaining metal particles consisting essentially of palladium and goldin close contact, wherein the first washcoat is positioned to encounterthe engine exhaust prior to the second washcoat, wherein the secondsupported catalyst has a palladium to gold weight ratio range of about0.5:1 to about 1:0.5.
 11. The emission control catalyst according toclaim 10, wherein the first washcoat further includes zeolite.
 12. Theemission control catalyst according to claim 10, further comprising athird washcoat containing zeolite.
 13. The emission control catalystaccording to claim 10, wherein the substrate has a honeycomb structurewith gas flow channels and multiple layers of washcoats are coated onthe walls of the gas flow channels, and wherein a bottom layer that isdirectly in contact with the walls of the gas flow channels includes thesecond washcoat and a top layer that is positioned to be in directcontact with the engine exhaust includes the first washcoat.
 14. Theemission control catalyst according to claim 13, wherein the multiplelayers include a middle layer between the bottom layer and the toplayer, and the middle layer includes a third washcoat containingzeolite.
 15. The emission control catalyst according to claim 10,wherein the substrate has an upstream zone and a downstream zone, theupstream zone being positioned to encounter the engine exhaust beforethe downstream zone, and wherein the first washcoat is coated on theupstream zone of the substrate and the second washcoat is coated on thedownstream zone of the substrate.
 16. The emission control catalystaccording to claim 10, wherein the substrate has an upstream monolithand a downstream monolith, and wherein the first washcoat is coated onthe upstream monolith of the substrate and the second washcoat is coatedon the downstream monolith of the substrate.
 17. An emission controlcatalyst comprising a first catalyst comprising platinum-containingmetal particles, a second catalyst comprising palladium-gold containingmetal particles other than platinum-palladium-gold metal particles, andphysically separate zones for each of the first catalyst and the secondcatalyst, wherein the second supported catalyst has a palladium to goldweight ratio range of about 0.5:1 to about 1:0.5.
 18. The emissioncontrol catalyst according to claim 17, further comprising a buffer zonebetween the physically separate zones for the first catalyst and thesecond catalyst.
 19. The emission control catalyst according to claim17, wherein the first catalyst comprises a platinum-palladium catalysthaving a Pt:Pd weight ratio of 2:1 to 4:1.
 20. The emission controlcatalyst according to claim 19, further comprising a zeolite mixturehaving beta zeolite and ZSM-5 zeolite having a weight ratio of about1:1.